1. Field of the Invention
The invention pertains generally to circuit breakers.
2. Description of the Related Art
A single pole circuit breaker is a device which serves to interrupt electrical current flow in an electrical circuit path upon the occurrence of an overcurrent in the circuit path. On the other hand, a multipole circuit breaker is a device which includes two or more interconnected, single pole circuit breakers which serve to substantially simultaneously interrupt current flow in two or more circuit paths upon the occurrence of an overcurrent in any one circuit path.
An exemplary, conventional single pole circuit breaker is depicted in FIGS. 1, 2A and 2B. As shown, the single pole circuit breaker 10 includes an electrically insulating casing 20 which houses, among other things, stationarily mounted terminals 30 and 40. In use, these terminals are electrically connected to the ends of the electrical circuit which is to be protected against overcurrents.
As is known, the casing 20 also houses a stationary electrical contact 50 mounted on the terminal 40 and an electrical contact 60 mounted on a contact bar 70. Significantly, the contact bar 70 is pivotably connected via a pivot pin 80 to a stationarily mounted frame 100. A helical spring 85, which encircles the pivot pin 80, pivotally biases the contact bar 70 toward the frame 100. A contact bar stop pin 90, mounted on the contact bar 70, limits the pivotal motion of the contact bar relative to the frame. By virtue of the pivotal motion of the contact bar 70, the contact 60 is readily moved into and out of electrical contact with the stationary contact 50.
An electrical coil 110, which encircles a magnetic core 120 topped by a pole piece 130, is positioned adjacent the frame 100. An electrical braid 140 serves to electrically connect the terminal 30 to one end of the coil 110. An electrical braid 150 connects the opposite end of the coil 110 to the contact bar 70. Thus, when the contact bar 70 is pivoted in the clockwise direction (as viewed in FIG. 1), against the biasing force exerted by the spring 85, to bring the contact 60 into electrical contact with the contact 50, a continuous electrical path extends between the terminals 30 and 40.
The circuit breaker 10 also includes a handle 160 which is pivotably connected to the frame 100 via a pin 170. In addition, a toggle mechanism is provided, which connects the handle 160 to the contact bar 70. As more clearly depicted in FIG. 2A, this toggle mechanism includes a cam link 190 which is pivotably connected to the handle 160 via a pin 180. A significant feature of the cam link 190, shown in expanded view in FIG. 2B, is the presence of a step, formed by the intersection of non-parallel surfaces 194 and 198, in the outer profile of the cam link 190.
With further reference to FIGS. 2A and 2B, the toggle mechanism of the circuit breaker 10 also includes a link housing 200, to which is connected a projecting arm 205. The link housing is pivotably connected to the cam link 190 by a rivet 210 and pivotably connected to the contact bar 70 by a pin 220.
The toggle mechanism further includes a sear assembly, including a sear pin 230 which extends through an aperture in the link housing 200 to the cam link 190. This sear pin includes a circularly curved surface 232 (see FIG. 2B) which is intersected by a substantially planar surface 233. The sear assembly also includes a leg 235 (see FIG. 2A), connected to the sear pin 230, and a sear striker bar 240, which is connected to the leg 235 and projects into the plane of the paper, as viewed in FIG. 2A. A helical spring 250, which encircles the sear pin 230, pivotally biases the leg 235 of the sear assembly into contact with the leg 205 of the link housing 200. As a consequence, the sear pin 230 engages the step in the cam link 190, i.e., a portion of the surface 194 of the cam link 190 overlaps and contacts a portion of the curved surface 232 of the sear pin 230. Significantly, it is by virtue of this engagement that the toggle mechanism is locked and thus capable of opposing and counteracting the pivotal biasing force exerted by the spring 85 on the contact bar 70, thereby maintaining the electrical connection between the contacts 50 and 60.
By manually pivoting the handle 160 in the counterclockwise direction (as viewed in FIG. 1), the toggle mechanism, while remaining locked, is translated and rotated out of alignment with the pivotal biasing force exerted by the spring 85 on the contact bar 70. This biasing force then pivots the contact bar 70 in the counterclockwise direction, toward the frame 100, resulting in the electrical connection between the contacts 50 and 60 being broken. Manually pivoting the handle 160 in the clockwise direction then serves to reverse the process.
As shown in FIG. 1, the single pole circuit breaker 10 also includes an armature 260, pivotably connected to the frame 100. This armature includes a leg 265 which is positioned adjacent the sear striker bar 240. In the event of an overcurrent in the circuit to be protected, this overcurrent will necessarily also flow through the coil 110, producing a magnetic force which induces the armature 260 to pivot toward the pole piece 130. As a consequence, the armature leg 265 will strike the sear striker bar 240, pivoting the sear pin 230 out of engagement with the step in the cam link 190, thereby collapsing the toggle mechanism. In the absence of the opposing force exerted by the toggle mechanism, the biasing force exerted by the spring 85 on the contact bar 70 will pivot the contact bar in the counterclockwise direction, toward the frame 100, resulting in the electrical connection between the contacts 50 and 60 being broken.
Two or more single pole circuit breakers 10 are readily interconnected to form a multipole circuit breaker. In this configuration, each such single pole circuit breaker 10 further includes, as depicted in FIG. 3, a trip lever 270 which is pivotably connected to the frame 100 via a pivot pin 320. The trip lever 270 is generally U-shaped and includes arms 280 (shown in FIG. 3) and 290 (not shown in FIG. 3) which at least partially enfold the frame 100. A helical spring 330, positioned between the frame 100 and the arm 280 and encircling the pin 320, pivotally biases the trip lever toward the frame 100. A projection 300 of the trip lever 270, which, as viewed in FIG. 3, projects out of the plane of the paper, is intended for insertion into a corresponding aperture in the trip lever of an adjacent single pole circuit breaker. Thus, any pivotal motion imparted to the trip lever 270, in opposition to the biasing force exerted by the spring 330, is transmitted to the adjacent trip lever, and vice versa.
In the operation of the single pole circuit breaker 10, when employed in a multipole circuit breaker, if an overcurrent flows through the coil 110, then, as a result, as described above, the single pole circuit breaker 10 will be tripped, i.e., the contact bar 70 will be pivoted in the counterclockwise direction and the electrical connection between the contacts 50 and 60 will be broken. During this pivoting motion, the pin 220, pivotably connecting the link housing 200 to the contact bar 70, will engage a camming surface 285 on the bottom of the arm 280, thereby applying a torque to the trip lever 270. Consequently, the trip lever 270 will be pivoted away from the frame 100 and toward the armature 260. This pivotal motion will also be imparted to the trip lever of the adjacent single pole circuit breaker via the projection 300. Provided the torque applied by the pin 220 is sufficiently large, then the trip lever of the adjacent single pole circuit breaker will depress the corresponding armature, thereby tripping the adjacent single pole circuit breaker.
While single pole circuit breakers of the type described above are definitely useful, there are certain difficulties associated with their manufacture. For example, as more readily understood with reference again to FIGS. 2A and 2B, one such difficulty is associated with achieving an appropriate amount of overlap between the surface 194 of the cam link 190 and the curved surface 232 of the sear pin 230, when the toggle mechanism is in the locked position. As is known, the minimum amount of overlap is dictated by the need to prevent the toggle mechanism from collapsing in the absence of an overcurrent, if the circuit breaker is merely mechanically vibrated. On the other hand, the maximum amount of overlap is dictated by the requirement that the toggle mechanism collapse, upon the occurrence of an overcurrent, no later than the instant the armature 260 strikes the pole piece 130. In fact, it is generally desirable that collapse occur as the armature 260 pivots toward the pole piece 130 but before it actually impacts the pole piece. This is due to the fact that if the overlap is chosen so that collapse occurs only at the point of impact, then manufacturing variability may result in overlaps which are slightly larger than desired, and in corresponding toggle mechanisms which do not collapse even at the point of impact.
Conventionally, to achieve an appropriate amount of overlap., the depth of the step, i.e., the length of the surface 194, of the cam link 190 is made significantly greater than the desired overlap. In addition, during the process of assembling the toggle mechanism, an assembly-line worker bends the leg 205 of the link housing 200 to different test positions, resulting in the biasing spring 250 rotating the curved surface 232 of the sear pin 230 into different overlaps with the surface 194. At each test, position, a shim of specified thickness is inserted between the leg 235 of the sear assembly and the arm 205 of the link housing, which is intended to cause toggle collapse. Once collapse occurs, the correct amount of overlap is assumed to have been achieved. This procedure is also repeated after assembly of the circuit breaker as a whole. The resulting length of the overlap, expressed as a percentage of the length of the surface 194, is, at most, 75 percent, and usually significantly less than 75 percent.
Obviously, the current procedure for achieving appropriate amounts of overlap is time-consuming, requires manual labor and, as a consequence, significantly adds to the cost of manufacturing and assembling circuit breakers.
Yet another difficulty associated with the manufacture of conventional circuit breakers is that of achieving uniform eccentricities. That is, when the toggle mechanism of the circuit breaker 10 is in the locked position, as depicted in FIG. 2A, and an imaginary, straight line 215 is drawn from the center of the pivot pin 180 to the center of the pivot pin 220, then the center of the rivet 210, which pivotably connects the cam link 190 to the link housing 200, should be to the right (as viewed in FIG. 2A) of the imaginary line 215. (If the center of the rivet 210 were to the left of the imaginary straight line 215, then rotating the sear pin 230 out of engagement with the step in the cam link 190 would not lead to the collapse of the toggle mechanism.) The length of a perpendicular 217 extending from the imaginary line 215 to the center of the rivet 210 is defined as the eccentricity of the toggle mechanism. Significantly, the magnitude of the force that must be applied to the sear striker bar 240 to collapse the toggle mechanism is determined by the eccentricity, i.e., the larger the eccentricity, the larger the force, and vice versa.
If the toggle mechanism is initially in the collapsed position, then pivoting the handle 160 in the clockwise direction (as viewed in FIG. 1) will produce translation and rotation of the toggle mechanism components into the locked position, shown in FIG. 2A. In particular, during this translation and rotation, the cam link 190 initially undergoes a relatively small amount of rotation about the rivet 210 in the clockwise direction (as viewed in FIG. 2A) until the back of the cam link 190 contacts the inner surface of the link housing 200. The length of the perpendicular 217 relative to the line 215, at the point when the cam link 190 achieves its maximum clockwise rotation, is here termed the baseline eccentricity, which is largely determined by the basic geometry of the circuit breaker configuration. After reaching its position of maximum clockwise rotation, the cam link 190 undergoes counterclockwise rotation about the rivet 210 until the surface 194 of the cam link 190 contacts the curved surface 232 of the sear pin 230, thereby locking the toggle mechanism. As a result of this counterclockwise rotation, the baseline eccentricity is increased by an amount here termed the supplemental eccentricity, to arrive at the eccentricity, as defined above. But the amount of counterclockwise rotation is related to the initial amount of clockwise rotation, which is limited by the distance between the back of the cam link 190 and the inner surface of the link housing 200. However, the link housing 200 is formed by bending sheet metal, a relatively imprecise process which introduces considerable variability into the distance between the back of the cam link 190 and the inner surface of the link housing 200, and a corresponding variability into the supplemental eccentricity, and thus into the eccentricity.
To minimize the eccentricity variability, described above, and thereby achieve uniformity of circuit breaker operation, it has been necessary to employ relatively large baseline eccentricities, which has led to relatively large eccentricities. However, the use of these relatively large eccentricities has led to toggle mechanisms which must be impacted by relatively large forces to achieve toggle collapse. Unfortunately, this implies that the corresponding circuit breakers are relatively insensitive to overcurrents.
To achieve meaningful comparisons of the eccentricities employed in circuit breakers having different physical dimensions, it is generally more useful to compare values of the ratio of the length of the imaginary straight line 215 to the corresponding eccentricity, a non-dimensional number hereinafter denoted R. Clearly, the smaller the eccentricity, the larger the value of R, and vice versa. Because of the need to employ relatively large values of eccentricity in conventional circuit breakers, as discussed above, the corresponding values of R have been limited to no more than about 47. Clearly, larger values of R, resulting from smaller eccentricities, are desirable as this would lead to circuit breakers which are more sensitive to overcurrents.
Thus, those engaged in developing circuit breakers have sought, thus far unsuccessfully, circuit breakers in which desired overlaps are more conveniently achieved, and which exhibit relatively large values of R.